8 PROBLEMS IN PHOTOSYNTHESIS 



It is customary to express chlorophyll concentrations in yumole, jA or mg. 

 Table 1 shows the conversion factors for these units. The molecular weight 

 of chlorophyll a (C55H7o05N4Mg) is 892. 



§ 3 The Structure of the Chloroplast 



The chloroplasts containing the chlorophylls and carotenoids vary in size 

 and number from plant to plant.* In algae each cell may contain just one 

 chloroplast, whereas in higher plants there may be several hundred per cell. 

 The size of the longest axis varies from 2 to 10 /x. The chloroplasts are the 

 site of the photochemical part of photosynthetic activity. In any case, 

 there is no evidence to the contrary or no reason to assume that cytoplasm 

 may also play an active part. Nevertheless, Hill and Whittingham (20), 

 citing older work by Frenkel (13), assume that COo-fixation in photosynthesis 

 may be associated with cytoplasm. It is difficult to accept such a hypothesis 

 since an experimentally well-established stoichiometric relationship exists 

 between CO2 and chlorophyll (see § 59). 



The chloroplasts possess a membrane about 50 to 100 A thick (39). The 

 chlorophyll is localized in fairly small units — the grana — which are embedded 

 in the stroma. Grana and stroma are composed of proteins and lipoids or 

 lipoproteids. It has been observ^ed that the grana have a lamellae structure 

 probably surrounded by a membrane. Studies by Steinmann (36) with the 

 electron microscope show that the structure of intact chloroplasts of various 

 algae is lamellated throughout (see §51). In such chloroplasts (e.g., Spirogyra) 

 the lamellae are present in bundles between which thin layers of stroma occur 

 (40, 41). From this it follows that distinction has to be made between two 

 groups of chloroplasts. In the granulated chloroplasts, prevalent in higher 

 plants, the lamellated structure is limited to the grana. Thomas et al. (42) 

 propose the name chloroplast be given to this group only and that the lamel- 

 lated chloroplasts be simply called grana. The monolayer state of chlorophyll 

 in living chloroplasts has also been proposed by Trurnit and Colmano (43). 



According to Frey-Wyssling (14), the chlorophyll-containing lamellae are 

 embedded in lipoid layers which are situated between protein-water layers. 

 Hubert (21) and Wolken and Schwertz (49) have observed similar layer 

 structures. Figure 5 shows the geometry of the chloroplast as described by 

 Wolken and Schwertz. Baas Becking and Hanson (1) arrived at the con- 

 clusion that four chlorophyll molecules form a tetrade in such a way that the 

 isocyclic rings are turned towards each other. It follows from Figure 5 that 

 there is space for one carotenoid molecule per three chlorophyll molecules. 

 This represents a weight ratio chlorophyll/carotenoids of 5 to 1 . However, 

 the spaces between the chlorophyll molecules (15 A X 15 A) allow enough 

 room for four carotenoid molecules, the diameter of which is only 6 to 7 A. 

 The observations, analyses and calculations of Wolken and Schwertz concern 



* For more detailed information on chloroplast structure see Granick (16), Miihlethaler (27), 

 Wolken and Schwertz (49). 



